YOUR BEST PRACTICE


How well do you know your soil?


Identifying soil water characteristics for


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BY DAVE GOORAHOO, PHD


Fresno State Soils Judging Club student Jagdeep Basi learns about identifying a soil’s textural and structural properties.


M


any factors impact your ability to irrigate effectively — from equipment to water supply to scheduling method. But one of the most fundamental requirements to


efficient and effective irrigation is to know the texture and structure of soils in your field.


Water infiltration into the soil profile and flow within it, be it for agricultural purposes such as plant uptake or for environmental concerns such as nitrate leaching, is dependent on the soil water content and the energy status or tension at which the water molecules are held by the soil. The quantity of water within soil pores is referred to as the soil water content and can be expressed on mass or gravimetric basis, volumetric basis or depth equivalent (water depth) basis. The energy level or matric potential within the unsaturated zone of the soil, which is the region that is most


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 


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Figure 1: This soil water retention curve shows the major components that affect plant water uptake. AW = available water, FC = field capacity and WP = wilting point


26 Irrigation TODAY | Winter 2020


relevant to plant roots, is primarily attributed to the attraction of water molecules to the solid soil matrix.


Soil water characteristics


A soil water retention curve is the relationship between the water content and the soil matric potential (see fig. 1). This curve is unique for different types of soil, and it can be used to deter- mine the soil-water related properties such as the water content at saturation, field capacity and wilting point when the soil matric potential values are at zero, –1/3 bar and –15 bars, respectively.


Free water moves spontaneously into dry soil and, in so doing, the water loses energy. By convention, the standard state of free water


10,000


1,000


100 Sand 10 Clay Loam 1 Medium Clay Peat


0.1 0 0.2 0.4 θ (m3 m–3 0.6 )


Figure 2: Soil water retention curves for various soil types


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Ψ  Ψ   θ θ θ 


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|| (kPa)


Photo credit: Dave Goorahoo


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